A. Field of the Invention
The present invention relates to a semiconductor device used in, for example, a power conversion apparatus, and more particularly, to an insulated gate semiconductor device (IGBT) and a method for manufacturing the same.
B. Description of the Related Art
A technique for reducing the power consumption of a power conversion apparatus has been developed and there are great expectations for a power device capable of reducing the power consumption of the power conversion apparatus. Among the power devices, an insulated gate bipolar transistor (hereinafter, referred to as an IGBT) which obtains a low on-voltage using a conductivity modulation effect and easily controls a gate using a voltage has been generally used.
In a trench gate IGBT in which a trench with a small width is vertically formed from the surface of a silicon wafer and a gate electrode is provided in the trench with an oxide film interposed therebetween, channels are formed on both side surfaces of the trench. Therefore, it is possible to increase the density of channels and reduce an on-voltage, as compared to a so-called planar IGBT in which a gate electrode is formed on the surface of a silicon wafer. In recent years, the field of application of the trench gate IGBT has been expanded.
Next, the structure of the trench gate IGBT will be described. FIG. 4 is a cross-sectional view illustrating a main part of a general IGBT including a trench gate. FIG. 4 illustrates the cross section of a silicon wafer when an n-channel trench gate IGBT in which a gate electrode is provided in a trench with a stripe-shaped planar pattern (not illustrated) with a gate insulating film interposed therebetween is cut in a direction in which the trench gate is traversed in a plan view.
The trench gate IGBT illustrated in the cross-sectional view of FIG. 4 includes a p-type silicon substrate 1a with high concentration. An n-type silicon wafer, which is an n− drift layer 2 with low concentration, is provided on the surface of silicon substrate 1a. P base region 3 is formed on the surface of the silicon wafer. N+ emitter region 4 is selectively formed in the surface of p base region 3.
Trench 10 is formed so as to extend from the surface of n+ emitter region 4 to n− drift layer 2 through p base region 3. Gate insulating film 5 is provided on the inner surface of trench 10. Gate electrode 6 is filled inside gate insulating film 5. Gate electrode 6 is made of conductive polycrystalline silicon. In FIG. 4, gate electrode 6 is hatched. Gate insulating film 5 is interposed between the inner surface of trench 10 and gate electrode 6.
Interlayer insulating film 7 is formed so as to cover the upper part of gate electrode 6. Emitter electrode 8 is provided on the upper part of interlayer insulating film 7. Emitter electrode 8 has a sheet shape and is provided so as to contact both n+ emitter region 4 and p base region 3. Emitter electrode 8 is provided so as to cover p base region 3, the surface of n+ emitter region 4, and interlayer insulating film 7. In some cases, a nitride film or an amorphous silicon film is formed as a passivation film on emitter electrode 8. However, FIG. 4 does not illustrate the passivation film. In addition, a collector electrode 9 is provided on a surface (hereinafter, referred to as a rear surface) of p-type silicon substrate 1a opposite to n− drift layer 2.
Next, the operation of the IGBT will be described. However, p-type region 11 which is a surface layer region of the silicon substrate interposed between trenches 10 and is covered with emitter electrode 8 with insulating film (interlayer insulating film 7) interposed therebetween will be described below. First, an operation of changing the trench gate IGBT from an off state to an on state will be described.
The IGBT is turned off when emitter electrode 8 is generally connected to the ground, a voltage higher than emitter electrode 8 is applied to the collector electrode 9 (a forward voltage is applied), and a voltage applied to the gate is lower than a threshold value. When a gate driving circuit (not illustrated) applies a voltage higher than the threshold value to gate electrode 6 through a gate resistor, charge starts to be stored in gate electrode 6.
At the same time as charge is stored in gate electrode 6, a portion of p base region 3 which faces gate electrode 6 with gate insulating film 5 interposed therebetween is inverted into an n type to form a channel region (not illustrated). In this way, an electronic current is injected from the emitter electrode 8 to n− drift layer 2 through n+ emitter region 4 and n-channel region of p base region 3.
The junction between p-type silicon substrate 1a and n− drift layer 2 is forward biased by the injected electron and holes are injected from collector electrode 9. Then, the IGBT is turned on. In the IGBT which is in the on state, a voltage drop between emitter electrode 8 and collector electrode 9 is an on-voltage. At that time, the conductivity of n− drift layer 2 is modulated by the injection of the holes. Therefore, the IGBT has a low on-voltage, as compared to a MOSFET which has the same structure as the IGBT except that holes are not injected.
The voltage between emitter electrode 8 and the gate electrode 6 is set to be lower than a threshold value in order to change the IGBT from the on state to the off state. Then, the charge stored in gate electrode 6 is discharged to a gate driving circuit through a gate resistor. At that time, the n channel region which has been inverted into the n type returns to the p type and the n-channel region is removed. Therefore, no electron is supplied and no hole is injected. The electrons and holes stored in n− drift layer 2 are discharged (emitted) to collector electrode 9 and emitter electrode 8, respectively, or are recombined with each other. As a result, a current is reduced and the IGBT is turned off.
Various improvement methods have been proposed in order to further reduce the on-voltage of the trench gate IGBT. For example, an injection enhanced gate bipolar transistor (IEGT) disclosed in the following JP 5-243561 A (FIG. 101) has limitation characteristics close to the on-voltage of a diode.
In the IEGT, the surfaces of an n+ emitter region and a p base region are partially covered with an insulating layer and the covered region does not contact an emitter electrode. The basic operation of the IEGT is the same as that of the trench gate IGBT. Holes below the p base region in a portion of the IEGT in which the n+ emitter region and the p base region do not contact the emitter electrode are less likely to be emitted to the emitter electrode. Therefore, the holes are stored in the portion.
As a result, since the carrier concentration distribution of the n− drift layer is close to the concentration distribution of the diode, the on-voltage of the IEGT can be lower than that of the general trench gate IGBT. However, the power device requires high-speed switching characteristics, in addition to the low on-voltage and it is also important to improve the high-speed switching characteristics.
In the trench gate IGBT and the IEGT, the trench gate has a high-density structure in order to reduce the on-voltage. Therefore, the capacitance between the gate electrode and the emitter electrode increases. As described in the operation of the IGBT, when the IGBT changes to an on operation and an off operation, the capacitance between the gate electrode and the emitter electrode needs to be charged and discharged. However, when the capacitance between the gate electrode and the emitter electrode is large, the charge and discharge time increases and loss caused by the increase of the charge and discharge time in the charge and discharge time increases.
The loss of the power device is the sum of steady loss determined by the on-voltage and switching loss generated during the on operation and the off operation. Therefore, it is also important to reduce the switching loss. In order to reduce the switching loss, it is also necessary to reduce the capacitance between the gate electrode and the emitter electrode causing the switching loss.
Next, the structure of an IGBT disclosed in the following JP 2001-308327 A (FIG. 1) will be described again with reference to FIG. 4. In the IGBT, p-type region 11 is covered with insulating layer 7 so as not to contact emitter electrode 8, which makes it difficult for holes to be emitted to an emitter electrode. As a result, holes are stored in the vicinity of p-type region 11 and the carrier concentration distribution of n− drift layer 2 is close to that of the diode. In addition, since p-type region 11 is covered with insulating layer 7, the function of trench gate 10 does not operate effectively. As a result, the capacitance between gate electrode 6 and emitter electrode 8 is reduced and the charge and discharge time is shortened. Therefore, switching loss is reduced.
However, the structure of the IGBT (FIG. 4) disclosed in JP 2001-308327, that is, the structure including p-type region 11 which is interposed between the trenches, is insulated from emitter electrode 8, and is in the floating state in terms of potential has the essential problem that it is difficult to obtain a high breakdown voltage. The reason is as follows. Since the trenches are not arranged at regular intervals and p-type floating region 11 is provided between the trenches, the distribution of the electric field when the IGBT is turned off is not uniform, as compared to the structure in which p-type floating region 11 is not provided between the trenches, the electric field is likely to be concentrated on the bottom of the trench gate, and the breakdown voltage is likely to be reduced.
As means for solving these problems, a method has been known which forms p-type region 11 which is in a floating state in terms of potential to be deeper than trench 10, reduces the concentration of the electric field on the bottom of trench 10, and obtains a high breakdown voltage. However, in this method, since p-type region 11 which is in a floating state in terms of potential is formed deep, the effective thickness of n− drift layer 2 is reduced and the electric field strength in the vertical direction increases. Therefore, the breakdown voltage is likely to be reduced and it is difficult to sufficiently improve the breakdown voltage.
The present invention is directed to overcoming or at least reducing the effects of one or more of the problems set forth above.